On the contrary, I'm completely in favor of custom CSS scrollbars because it's unimaginable that websites could do worse than modern OSes already do when it comes to scrollbars. If anyone here has a time machine, immediately after you've finished killing Hitler may I recommend that you gently persuade Jony Ive away from the terrible decision to make scrollbars hidden, low contrast, and impossibly thin.
What I prefer is consistency with the OS. My OS is my main tool for interacting with the digital realm. My life would be easier if the conventions were more evenly applied. Custom scrollbars concern me that my OS defaults are being overriden: will the field scroll at the expected speed; can I flick it and catch it as I’m used to; is the scale expressive of content size?
Custom scrollbars tell me that the developer doesn’t care about me, they care more about their brand manager and their pay check. This suggests the product they represent wont suit me.
Along with intrusive pop-ups, subscription overlays, and “we value your privacy”, it’s a clear sign I should not engage with that provder.